1. Field of the Invention
This invention relates to multiple holographic lenses and more particularly to methods for fabricating a coherently exposed multiple holographic lens.
2. Description of the Prior Art
Multiple holographic lenses are used in optical matched filter correlators to accommodate parallel processing of an input scene with an array of matched filters. The matched filter array represents the stored memory of the correlator and contains information relating to different aspects of the specific target of interest, such as, size and orientation. The optical correlator system detects the presence of an object in a scene or field of view by finding a match between one or more of the images of the selected object stored in the matched filter with the various objects in the scene. In operation, a collimated light beam is spatially modulated by the input scene and directed through a multiple holographic lens. The output of the multiple holographic lens is a matrix of individual beams each of which is spatially modulated with the input scene. All the beams converge towards and pass through individual matched filters in the array for parallel processing by the stored memory array. If a match occurs, the output beams of the matched filter will be of sufficient intensity to produce a bright spot on a light sensitive detector that generates a signal indicating the presence of the selected target.
Traditionally, multiple holographic lenses have been made by what is known as the step and repeat method. A collimated reference beam and a point source object beam are superposed on a recording medium in accordance with well known holographic recording techniques. Upon exposure, the interference pattern of the reference and object beams is recorded thereby forming a holographic lens element on the recording medium. After the first exposure, the medium is moved a small amount in either the x or y direction and a second exposure is made of the superposed object and reference beams. This process is repeated several times until the desired pattern of multiple exposures is made as the medium is translated. Individual holographic lens elements are created by each of the exposures. The multiple holographic lens made in this manner, having an array of independently recorded lens elements, is known as an independently exposed multiple holographic lens.
Several deficiences in operating performance of the independently exposed multiple holographic lens are present. First, the total intensity of light recorded by the lens is the sum of contributions from each individual lens element. Since the recording medium has a finite contrast range, the intensity of the reference and object beams of each exposure must be reduced to avoid overexposing the medium. The reduction in recording beam intensity results, however, in a reduction in intensity of the playback beam. Secondly, the diffraction efficiency (DE) of the array of lens elements decreases as the number of lens elements in the array (M) increases, approximately in accordance with the formula DE=1/(M.times.M). This feature is a consequence of the additive nature of the intensity recording and finite contrast range of the recording medium. Thirdly, the DE of individual lens elements in the array are not necessarily equal. This lack of uniformity is due to the nonlinear characteristics of the recording medium.
U.S. Pat. No. 4,421,379, assigned to the same assignee as the present invention, discloses a two step process for fabricating a multiple holographic lens wherein the final holographic recording is made with a single exposure. In the first step, all the exposures to be recorded in the final lens are recorded individually in a modified form of the step and repeat method. In this modified form, a mask is placed on the photographic plate to prevent overlap of the exposure areas of the individual lens elements. In the second step, the first multiple lens is illuminated giving rise to divergent refracted beams that interfere with a second beam of light on a second photographic plate to produce the final multiple holographic lens. In this method, while the diffraction efficiency is increased by eliminating cross product distortion terms that may be present when the interference pattern of each lens element overlaps, the first holographic recording suffers from a non-uniformity since small portions of the plate are more sensitive to imperfections. In addition, the effective area of the final multiple holographic lens is limited by the high f-number required for fabrication of the first holographic lens.